With all the electronics you work with you surely have a lab power supply no? You can use that to charge the car battery. Also why not use one of your family’s batteries and tell them that you buy them a new one if you destroy the original - which you will not have to do anyway.
Sure I do, it’s a DIY MCU-controlled with quite a few useful functions…
…but I havn’t implemented battery charging alghoritms. I could do that, but then… yeah, I’m lazy. The firmware is already a mess ( I was learning C when I was coding it, it was one of my first projects that used microcontroller ) and I would have to rewrite the whole code for the device that I’m going to replace with the new, better one I’m working on for 7 months ( yes, I’m really, really lazy ) anyway… I will probably implement this in my new lab bench power supply once its finished, but that’s going to take quite a while as it’s not my priority right now.
And charging it manually ( aka watching if it’s charged and disconnect it ) is not something I can afford to do. Proper car battery charger is something I should have anyway.
Already tried, I was told to buy a new one and use it.
In the end, I might end up using the car battery for multiple projects so let’s just buy one 
Anyway, thanks for your help and advices, I appreciate you’re trying to help!
So, I was able to get an old car battery for free from my friend - hopefully it will do the job. I also bought 4A car battery charger for 25 euros. And finally, 40 Samsung 25R cells + some nickel strips from nkon.nl ( they really have great prices! ).
The pre-built welder @rwxr mentioned looks nice, but it’s not available. So I’m going to design my own as a plan B.
Will keep you updated 
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I’m going to this route.
But adding a push button like this guy
Easiest and most affordable spot welder ever
Geez, just saw the first video…
No timing circuit Too thin wires ( you should use 16mm2 or more ) Welding electrodes in a WOOD?
Geeez. Just… woooow. That’s so bad… Who on the Earth would do it like that? That’s insane.
And the result? Those welds are terrible! He is literally making holes in the nickel strip. Geeeeeeez.
I have designed simple spot welder. It uses arduino nano for timing, and solid state relay to activate the primary winding of the transformer.
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So, I finished the design, this time actually in a CAD. I changed my mind and instead of using mini oled display, I’m gonna stick with 7 segment. It draws much more current than the OLED display, but after some testing, I know 7805 can still handle it. It will also make my life easier during the programming part of the development.
Missing components are ordered ( I already had most of the components at home ).
There is the schematic:
I’m still not sure if I’m going to make a PCB for the whole thing or keep it simple and make a PCB just for the power switching part while keeping the logic part on the breadboard. Hmm… I’m gonna think about it once I start to design the PCB ( probably tomorrow ).
For some mortals as us, these diagrams are pretty difficult to follow. So, I’ll stick with lead acid spot welding method shown on the previous youtube video, not the first but the last one.
It’s actually very simple diagram! You have an programmed microcontroller ( I will open source the firmware as well ), that reacts to 2 inputs: button and pedal. Then, there is a shift register, which stores data about which segments of the display are active and also the one that enables the transistors that enables mosfets which then just create a path for the welding current. Then there is just a 5V linear regulator ( 7805, no reason to use buck converter there ) and a few passive components. Once I make it, I will post some pictures so you ( and others ) can see how simple it actually is!
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Did you also account for the TVS and Schottky Diode that are recommended to use with this type of spot welder?
No need for them as there is no inductive load and input is a car battery ( 12V, will drop during weld ).
Did you read the endless sphere topics about the arduino spot welder? There is quite a lot of avalanche current with a 12V battery. https://endless-sphere.com/forums/viewtopic.php?f=14&t=83411 https://endless-sphere.com/forums/viewtopic.php?f=14&t=81400&start=550#p1218325
I did not until now, but looks like they tested it in real world and found use for the diodes. From my calculations, the inductance should not be high enough to cause such a problems, also my real world tests on a prototype showed no problems, but okay, I will add these diodes just to be on a safe side.
I’m also going to improve the mosfet switching stage to something that can supply higher currents, because I found out that the mosfet switch time is too long.
Another problem I came up is the capacitance. 5V rail is super stable ( it has to be with 100u cap ), but the 12V rail falls before the mosfets get fully switched on. I will add high capacity capacitor to the new mosfet switching stage to keep the 12V rail stable enough during the weld.
I will implement all the changes soon.
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Just a quick update.
I have hard time looking for the diodes. Mostly because of “Stock level: 0 pcs” which is driving me crazy. Even if I find something that suits my needs, it’s not in stock. I may look for it somewhere else, but ordering components from multiple distributors that I have no experience with is not really something I want.
List of available schottky diodes on TME.eu And the same for transil diodes.
Once I filter them, there is no diode in stock that would suit this project 
.
Any advice? Maybe I will just ignore the diodes.
By the way, I was able to find some mosfet drivers and they are quite cheap. Two of these 4A 2-channels should get the job done.
I still need to source better electrodes somewhere. Right now, I’m using 2.5mm2 solid core copper wire, I was unable to source anything thicker. It works, but it’s not that great… 10mm2 solid core would be much better. I visited 7 ( !!! ) local electronics stores and none of them had anything bigger than 2.5mm2 so that might be hard to source as well.
There are diodes in stock. The recommended ones are: https://endless-sphere.com/forums/viewtopic.php?f=14&t=81400&start=550#p1219916
and TME.eu has for example:
the original design uses these mosfet drivers:
What about the other method?
